Disk drives, also called disk files, are information storage devices that use a rotatable disk with concentric data tracks containing the information, a head or transducer for reading and/or writing data onto the various tracks, and an actuator connected to a carrier for the head for moving the head across the disk. A servo control system receives prerecorded servo positioning information read from the disk by the head, and sends control signals to the actuator to move the head to the desired track and maintain it over the track centerline. There are typically a number of disks mounted on a hub that is rotated by a spindle motor, and a number of head carriers for accessing the surfaces of the disks. The head carriers are supported on arms attached to the actuator.
Disk drives have undergone a rapid reduction in size. There are now commercially available disk drives that use disks with a 1.3-inch diameter. As the trend to reduced size continues, it becomes necessary to find smaller and easily manufacturable components. Because the conventional spindle motor is relatively complex to build on a small scale and is not readily capable of batch fabrication, the need arises for a new type of spindle motor that can be used with very small disk drives.
Micromechanical wobble motors, also called electrostatic harmonic motors, have been built on the scale of several hundred microns using semiconductor or thin film fabrication processes. The version of the wobble motor referred to here is one in which the static and movable elements remain coplanar during operation. The wobble motor provides planetary or eccentric rotational motion of an output shaft that is located within or surrounds a stator having a plurality of circumferentially-spaced stator elements. As the stator elements are sequentially electrostatically activated with respect to the grounded rotor, the rotor rolls around the stator. Because one complete electrical cycle of all of the stator elements causes the rotor to make much less than one complete revolution, the relatively high ratio of stator electrical cycles to rotor output shaft revolutions generates a high output torque. The high output torque of wobble motors makes them likely candidates for disk drive spindle motors to drive small (subcentimeter) diameter disks. However, the planetary motion of the output shaft renders them unusable as disk drive spindle motors because this motion is not compatible with conventional disk drive servo control systems.
What is needed is a micromechanical motor that exploits the batch fabrication and high torque advantages of the wobble motor, but which provides the rotary motion necessary for a disk drive spindle motor, rather than the conventional planetary motion.